The foamed insulation layers of foamed electric wires used in high-speed transmission cables such as USB 3.0 cables, HDMI cables, InfiniBand cables or micro USB cables are required to have a narrow diameter, have high heat resistance and enable the formation of fine foam.
A foamed insulation layer obtained by melting a polypropylene-based resin and supplying a chemical foaming agent such as azodicarbonamide thereto followed by uniformly kneading is conventionally known to be such a foamed insulation layer (Patent Document 1).